1. Field of the Invention
This invention relates generally to the field of a microelectromechanical system (MEMS), and in particular to an electrostatic MEMS device including one or more plates mechanically coupled to a mirror or the like to increase the electrostatic forces for actuating the mirror.
2. Description of the Related Art
MEMS devices offer significant advantages over conventional electromechanical systems with respect to their application, size, power consumption and cost of manufacture. Moreover, leveraging off of the significant progress in the manufacture of integrated circuits on silicon substrates over the past two decades, MEMS devices may be batch processed and packaged together with other IC devices using standard integrated processing techniques and with minimal additional processing steps.
While MEMS devices may be micromachined according to a variety of methodologies, a MEMS device may be formed by applying a thin film layer on a substrate, covering the film with a layer of photoresist, masking the photoresist in the pattern of the desired device features for that layer, and then etching away the undesired portions of the thin film layer. This deposition and photolithographic definition process may be repeated to apply successive etched thin film layers on the substrate until the micromechanical device is formed. A final release etching step is typically performed which removes material from within and around the micromechanical device to release the device so that it can perform its mechanical function. Electrical connections are often also made to the device to allow controlled movement of, or sensing through the device. The materials from which the layers are formed are selected to control the mechanical, electrical and/or chemical response of the layer and overall device.
One type of MEMS device is a parallel plate electrostatically-actuated mirror for use in an optical switching array or the like. Such a device, represented schematically in FIG. 1A, in general comprises a pair of spaced apart plates or electrodes 20 formed on the substrate 22, with one being stationary and the other being cantilevered so that the free end pivots toward or away from the fixed plate. The movable plate is coated with a reflective material, such as for example gold, to act as the mirror.
A known voltage potential V is applied across the electrodes 20, which voltage generates an electrostatic attractive force between the electrodes. Depending on the modulus of elasticity of the flexible electrode and the electrostatic force generated across the electrodes, the free end of the flexible electrode will move a fixed distance to accomplish some associated mechanical actuation.
In particular, referring to FIG. 1B, for a flat plate with a flexible suspension and ignoring fringing effects, the electrostatic bending force, or moment, Me generated in the top, flexible electrode 20 is given by the relationship:             M      e        =                  1        2            ⁢                        (                                    ∂              C                                      ∂              θ                                )                ·                  V          2                      ,
where C is the capacitance between the electrodes and V is the applied voltage potential across the electrodes. The change in capacitance with a change in the angle xcex8 is approximated by:                     ∂        C                    ∂        θ              =                            -          ϵ                ⁢                  W                      θ            2                          ⁢        ln        ⁢                  xe2x80x83                ⁢                  (                      α                          α              -              θ                                )                    +              ϵ        ⁢                  W                      θ            ⁡                          (                              α                -                θ                            )                                            ,
where xcex5 is the electrical permitivity of the dialectric (generally air) between the electrodes, W is the width of the electrode, and xcex1 is the ratio of the initial gap to the electrode length (g0/L). Thus, the electrostatic bending force Me exerted on the flexible electrode can be expressed as:       M    e    =                    -                  ϵ          2                    ⁢                        W          ⁢                      xe2x80x83                    ⁢                      V            2                                    θ          2                    ⁢      ln      ⁢              xe2x80x83            ⁢              (                  α                      α            -            θ                          )              +                  ϵ        2            ⁢                                    V            2                    (                      W                          θ              ⁡                              (                                  α                  -                  θ                                )                                              ⁢                      xe2x80x83                    )                .            
Thus, the flexible electrode in a MEMS actuator will displace through an angle xcex8 upon introduction of an actuation voltage depending on the magnitude of the voltage, the capacitance of the electrodes and mechanical properties of the flexible electrode.
A characteristic to electrostatically-actuated mirrors of the type described above is that, at actuation voltages and/or displacements above a threshold level, the electrostatic force between the electrodes becomes too strong and the flexible electrode collapses against the fixed electrode, a phenomena referred to as xe2x80x9cpull inxe2x80x9d. It has been analytically determined that pull down occurs at a voltage causing a displacement of:   θ  ≥                    0.44        ⁢                  xe2x80x83                ⁢                  g          0                    L        .  
Thus, where the voltage in the system shown in FIGS. 1A and 1B causes the flexible electrode to move through an angle xcex8 greater than 0.44 of the ratio of the initial gap to the length of the electrode, electrode pull in occurs. While it is known to provide an additional capacitor in series with the above electrostatic actuator to prevent electrode pull in, the maximum displacement is in any event limited to the initial gap length, which must be kept relatively small, generally on the order of 1 to 100 microns (xcexc), to avoid having to use excessively large actuation voltages.
Moreover, due to the relatively small size of the mirror and plates, and the fact that the plates must be relatively far apart to achieve large actuation angles, electrostatic actuation requires excessive voltages to achieve a satisfactory deflection of the mirror. Actuating a small mirror can require very large voltages, for example in excess of 200 volts. Such high voltages are difficult to generate and control using only small, low power semiconductor-based electronics.
It is therefore an advantage of the present invention to provide a system capable of actuating a mirror between at least two positions using lower voltages.
It is a further advantage of the present invention to generate relatively large actuation forces for moving a mirror between at least two positions using small, low power semiconductor-based electronics.
A still further advantage is that a mirror may be actuated through a relatively large angle using small, low power semiconductor based electronics.
It is another advantage of the present invention to provide a system capable of actuating a mirror between at least two positions which may be easily implemented using conventional micromachining techniques.
These and other advantages are provided by the present invention which in preferred embodiments relates to an electrostatic MEMS device for actuating a mirror or the like. The microactuator and mirror may be used for example as a bi-stable switch in an optical switching array. In order to accomplish switching between the two receivers, the microactuator is capable of actuating the mirror between two precisely repeatable positions. The mirror may be actuated to and between greater than two repeatable positions to achieve a plurality of optical switching conditions in alternative embodiments.
One embodiment of the microactuator includes first and second microcapacitors on either side of the mirror base plate. Each microcapacitor includes a stationary plate formed on a substrate, and a movable, actuation plate anchored to the substrate via microsprings and supported over the associated stationary plate. Upon application of a voltage to a microcapacitor, an electrostatic force is generated between the plates that causes the actuation plate of that microcapacitor to rotate toward the stationary plate. The actuation plates are in turn coupled to either side of the mirror base plate by microsprings between the actuation plates and mirror base plate. Thus, as an actuation plate rotates under a generated electrostatic force, torque from the actuation plate is transmitted as a downward force on the mirror base plate.
While the length of the actuation plates may vary in alternative embodiments, the plates may be two to ten times longer than the mirror length. For a given applied voltage potential, the large actuation plates increase the overall electrostatic force that may be applied to the mirror base plate and mirror.